Oriented polypropylene films are widely used in tape applications and in packaging applications such as food packaging. The optimization of processing characteristics and film properties of propylene based films has been the subject of intense effort. U.S. Pat. No. 5,118,566, for example, describes a biaxially oriented film made from polypropylene, a natural or synthetic resin, and a nucleating agent. The process for producing this film includes biaxially drawing the film at temperatures below the melting point of the polypropylene.
The present inventors have discovered crystalline propylene polymer compositions made by polymerizing propylene in one stage and then propylene and a minor amount of comonomer in a separate stage using a metallocene catalyst system comprising at least two metallocenes in each of the stages. The resulting polymers have surprisingly high molecular weight and broad molecular weight distribution, and offer processability benefits in oriented film applications. Films made from these unique polymers have a significantly broader processability range and can be evenly stretched at lower temperatures compared to the polypropylene films available today. The resulting films have a favorable balance of properties including high strength, good optical properties and good barrier properties.
Multiple stage polymerization processes are known in the art as is the use of multiple metallocenes, however, multiple stage polymerization processes are usually used to prepare block copolymers which contain rubbery materials as opposed to the crystalline polymers of this invention. U.S. Pat. Nos. 5,280,074; 5,322,902, and 5,346,925, for example, describe two-stage processes for producing propylene block copolymers. The propylene/ethylene copolymer portion of these compositions is a non-crystalline, rubbery material suitable for molding applications rather than films. U.S. Pat. No. 5,350,817 and Canadian Patent Application No. 2,133,181 describe the use of two or more metallocenes for the preparation of isotactic propylene polymers, however, neither reference describes a multiple stage process for the production of crystalline polymer compositions such as those described herein.